I found the solution by trying to understand the makefiles for the official examples.
Here is my Makefile :
OBJS=file1.cmo file2.cmo file3.cmo
NAME=projectname
OCAMLC=ocamlfind ocamlc -package js_of_ocaml -package js_of_ocaml.syntax -syntax camlp4o
$(NAME).byte: $(OBJS)
$(OCAMLC) -linkpkg -o $@ $(OBJS)
$(NAME).js: $(NAME).byte
js_of_ocaml $<
%.cmo: %.ml
$(OCAMLC) -c $<
...